function [J] = df(x1,x2);
J = zeros(2);
J(1,1) = 1      + 0.1*x2;
J(1,2) = 0.1*x1 + 0.4*x2;
J(2,1) = 2. *x1 + 1.5*x2;
J(2,2) = 1.5*x1 - 0.7;
end
% F1 = x1    + 0.1*x1*x2 + 0.2*x2**2 - 0.17975
% F2 = x1**2 + 1.5*x1*x2 - 0.7*x2    + 0.41125
